1. Field of the Invention
The present invention relates to a substrate transfer apparatus that is capable of properly preventing a retention arm from coming into contact with a substrate when the retention arm removes the substrate from a holder, which holds a plurality of semiconductor wafers or other substrates in a shelf form. The present invention also relates to a substrate transfer method and a storage medium for use with the substrate transfer method.
2. Description of the Related Art
A thermal treatment apparatus is used as a part of semiconductor manufacturing equipment to provide batch thermal treatment for a large number of semiconductor wafers (hereinafter referred to as “wafers”). As shown, for instance, in FIG. 16, the thermal treatment apparatus has a loading/unloading area Sa, which is used when a carrier 10 containing a plurality of wafers 1 is loaded from or unloaded to the outside. An automatic transport robot or operator loads the carrier 10 into the loading/unloading area Sa. A transfer apparatus 11 then transfers the wafers 1 in the carrier 10 to a wafer boat 12, which retains a large number of wafers 1 in a shelf form, and loads the wafer boat 12 into a thermal treatment furnace (not shown). A predetermined thermal treatment is then simultaneously provided for a large number of wafers 1.
The transfer apparatus 11 includes a base 14, which freely moves up and down, rotates around a substantially vertical axis, and moves in a substantially horizontal direction, and a plurality of forks 13, which freely move forward and backward along the base 14 and retain a large number of wafers 1 (e.g., five wafers 1). For example, five forks 13 simultaneously transport five wafers 1 between the carrier 10 and wafer boat 12 for transfer purposes. In this instance, the rim of each wafer 1 in the carrier 10 or wafer boat 12 is retained by a retainer (not shown) while a predetermined clearance is provided between the upper and lower neighboring wafers. Each of the five forks 13 is then inserted into a space beneath a wafer 1 retained by the carrier 10 or wafer boat 12. Next, each fork 13 is raised to lift the wafer 1 off the retainer to place the wafer 1 on each fork 13. Each fork 13 is then retracted to move the wafer 1 away from the carrier 10 or wafer boat 12. Subsequently, the transfer apparatus 11 transports the wafer 1 to its transfer destination.
The base end of each fork 13 is screwed down on an advance/retraction section 15. However, if, for instance, a fork 13 has been used for a long period of time or is improperly screwed down, it may look like the fourth fork 13 from the top (see FIG. 17). More specifically, the fork 13 may not be parallel to the horizontal plane or may become warped so that the leading end of the fork 13 is displaced upward or downward from its reference position.
The distance between the wafers 1 placed in the carrier 10 or wafer boat 12 is approximately 6 mm to 12 mm. Therefore, if the leading end of a fork 13 is inclined upward or downward as described above, a wafer 1 comes into contact with the fork 13 when the wafer 1 is received from the carrier 10 or wafer boat 12. Consequently, the surface of the wafer 1 may be scratched or abraded due to the contact with the fork 13. This may lead to the generation of particles.
Under the above circumstances, the wafers 1 are subjected to a surface defect inspection in which the surface of a wafer is checked, for instance, for damage or particle attachment. However, a conventional inspection apparatus for checking wafers 1 for surface defects is installed in an area apart from the thermal treatment furnace. Therefore, the wafers 1 are subjected to a predetermined thermal treatment and transported back into the carrier 10. The carrier 10 is then transported to the inspection apparatus to inspect target wafers in the carrier 10 in a predetermined manner.
If any particle, damage, or other defect is found on the surface of a wafer 1 in the above inspection process, the thermal treatment apparatus is brought to an emergency stop. The next step is performed to check whether the encountered defect is caused by the contact between a fork 13 and wafer, which is a result of upward/downward inclination of the fork 13. If so, appropriate maintenance tasks are performed, for instance, to adjust the position of the fork 13.
However, even if any surface defect is found in a situation where wafers 1 are inspected as described above after thermal treatment, the thermal treatment furnace performs a process not only on an already inspected wafer lot but also on the next wafer lot. Therefore, even if appropriate maintenance tasks are performed, for instance, to adjust the position of a fork 13 with the thermal treatment apparatus brought to an emergency stop, surface defects may be found on a large number of previously processed wafers to decrease the yield. Further, if any maintenance is performed on the fork 13 with the thermal treatment apparatus brought to an emergency stop, the apparatus cannot be operated during a period of time required for maintenance. This reduces equipment availability, thereby causing a decrease in productivity.
In recent years, the intervals at which the wafers on the wafer boat 12 are positioned tend to decrease in order to increase the number of wafers to be processed per batch with a view toward processing efficiency enhancement. In addition, the wafers tend to further increase in diameter. This increases the distance between the base end and leading end of a fork 13. Thus, it is anticipated that the inclination of the fork 13 from the horizontal plane may increase. As a result, the aforementioned problem has become evident.
As such being the case, the inventors of the present invention have conducted studies to work out a method of inspecting a fork 13 for up-down inclination before receiving a wafer 1 from the carrier 10, adjusting the position of the fork 13 to control the contact between the fork 13 and the wafer 1 in the carrier 10, and preventing the surface of the wafer 1 from being damaged. A technology for detecting the up-down inclination of a fork is proposed in JP-A-2005-51171.
The technology proposed in JP-A-2005-51171 is such that an optical sensor is mounted on a lower wall of a load lock chamber to measure the vertical distance to a blade of a wafer transport robot and measure the forward sag of the blade. However, the configuration disclosed in JP-A-2005-51171 can measure only one fork because the optical axis of the optical sensor is formed in vertical direction. Therefore, when, for instance, the employed transfer apparatus advances or retracts five forks simultaneously, the configuration disclosed in JP-A-2005-51171 cannot solve the aforementioned problem because it cannot inspect each fork for up-down inclination.
Meanwhile, the technology proposed in JP-A-2000-124290 detects the positional displacement of a wafer in a wafer cassette with a reflective photosensor and controls a robot hand in accordance with a value detected by the photosensor to prevent the robot hand from coming into contact with a wafer in the wafer cassette. However, this technology relates to the positional displacement of a wafer in a wafer cassette, and does not solve the aforementioned problem because it does not inspect a fork for up-down inclination before receiving a wafer from a carrier and adjust the position of the fork.